fix(Inference): Consider shunted metas when comparing equality (#555)

When comparing for equality of metavars, we need to consider that they
might have been merged to form a new meta, so
`UnificationContext::resolve` must be used. In this case, the variable
(LHS) is updated to the merged value in
`UnificationContext::merge_equal_metas` so we don't need to resolve that
one.

src/extension/infer.rs

@@ -664,7 +664,13 @@ impl UnificationContext {
                 let solution =
                     ExtensionSet::from_iter(self.get_constraints(&m).unwrap().iter().filter_map(
                         |c| match c {
-                            Constraint::Plus(x, other_m) if &m == other_m => Some(x.clone()),
+                            // If `m` has been merged, [`self.variables`] entry
+                            // will have already been updated to the merged
+                            // value by [`self.merge_equal_metas`] so we don't
+                            // need to worry about resolving it.
+                            Constraint::Plus(x, other_m) if m == self.resolve(*other_m) => {
+                                Some(x.clone())
+                            }
                             _ => None,
                         },
                     ));